1. Field of the Invention
This invention relates to the field of read/write head positioning schemes for rotating media storage systems.
2. Background Art
In a typical rotating medium storage system, data is stored on magnetic or magneto optic disks in a series of concentric or spiral "tracks." These tracks are accessed by read/write heads that detect variations in a magnetic orientation of the disk surface. The heads are mounted to an arm that is in turn mounted to an actuator motor. The arm may be pivotally mounted to the actuator motor, (much like the tone arm of a phonograph), or a linear actuator motor may be used so that the path of motion of the read/write heads is along a radius of the disk itself.
The actuator motor is typically a "voice coil" electrodynamic motor that has a coil moving within a permanent magnet, defining a cylindrical core. Alternatively, the motor may have a "rotary" type coil, such as is described in U.S. Pat. No. 4,805,055. The actuator motor is used to move the heads back and forth over the disk surface. The velocity of the heads as they move across the disk surface is dependent on the amount of current transmitted to the coil.
In the operation of a disk drive, it is often necessary to move the read/write heads from a current track to a desired track. This move is often between non-adjacent tracks and is referred to as a "seek" operation. In a seek operation, a command is provided to the disk drive to access a certain sector or sectors of information. If the heads are not currently over the track containing the desired sectors, a "seek profile" is determined. A seek profile describes the acceleration, deceleration, velocity and position information of the heads in moving from a current track to a destination track. The appropriate current is provided to the actuator motor to move the heads to the destination track, following the seek profile. Periodically, the actual position and/or velocity of the heads is compared to the seek profile. Adjustments are made to the current supplied to the actuator motor, if necessary, so that the heads follow the seek profile.
In the present description a track represents a radial location on the surface of one storage disk. A "cylinder" refers to the three dimensional path traced by the same radial position on a plurality of disk surfaces, such as in a multi-disk storage system.
When a data track has been accessed, it is important that the read/write head be kept on the center line of that track for accurate reading and writing operations. This positioning of the read/write head on the center line of a track is known as "track following." Variations from the center line of the track being followed produce a position error signal (PES) that is used to generate a corrective input to the head positioning apparatus to move the head back to the center line position.
Servo position information on either side of the center line of a data track is read and detected by the magnetic heads. A PES is generated and represents deviation of the magnetic head from the track center. A positive or negative PES indicates that the head is off center in one direction or the other, and suitable correction signals are generated.
Seeks and track following operations are performed under the control of a head positioning "servo" mechanism. Position information is provided through the use of special servo patterns recorded on the surface of the disk. A servo pattern is a permanent pattern pre-recorded on the storage disk at the time of assembly of the completed disk drive. The servo pattern represents position information such as track position information, sector number, index, etc. The servo pattern is detected by the head, and after appropriate signal processing, yields track position information. There are a number of methods of providing servo data in a disk drive, including "dedicated" servo and "sector" servo.
In a dedicated servo scheme, one entire surface of a disk contains servo information. A servo head accesses the servo surface of the servo disk to read the position information stored thereon. The servo head is in a fixed relationship relative to the read/write heads, so that the position of the servo head can be used to indicate the position of the read/write heads. A dedicated servo scheme is often used in disk drives having multiple disks. In a single disk system, a dedicated servo scheme is not practical, since 50% of the available storage area (one surface of the two sided disk) is unavailable for data storage. The disk surface area that is dedicated to servo tracks cannot be used for data tracks.
In the sector servo method, bursts of servo information are disposed on a disk surface in between data areas. Each servo burst contains track position information, track radial address fields and index information. Space division multiplexing of data and servo information minimizes track misregistration effects, since both data and servo information are reproduced from the same surface with a common head. The sector scheme is more efficient in single disk applications, since more disk surface area is available for data storage.
The head position is controlled by an actuator control system. The actuator control system obtains actual head position information from the servo pattern and compares it to desired head position information. When a position error is detected, the actuator control system provides a correcting current to the actuator motor to move the head to the desired position.
An example of a prior art actuator control system is illustrated in FIG. 1. FIG. 1 is a block diagram of a head position control loop. A desired track position 101, X.sub.track, is provided as an input to summing node 102. The summing node 102 is implemented as part of the disk drive actuator/head/disk assembly. X.sub.head 103, the actual head position, is provided to an inverting input of summing node 102. The difference between actual head position and desired track position is the output of summing node 102 and is X.sub.error 104.
The X.sub.error 104 is provided to head read/write electronics, pulse detect and demodulator block 105. Block 105 demodulates X.sub.error 104 and provides three output signals, cylinder number 106, the quadrature component 107 and the normal component 108 of X.sub.error 104. The normal component 108 is considered to be the PES. These signals are provided as input to a control microprocessor, generally indicated by dashed line 109. The microprocessor controls the three head positioning functions and operations; seeking, (indicated symbolically by seek block 110); error recovery, (indicated by block 111); and track following, (indicated symbolically by integrating block 112).
During seek operations, the seek block 110 is enabled and the seek is executed. During seek operations, the circuitry to the left of summing node 121 is disabled. The seek operation may be implemented by any of many well known control algorithms.
The error recovery block 111 is enabled when the heads are offset from a desired track by a large amount (on the order of several tracks). During error recovery, switch 113 is coupled to block 111 and the circuitry to the left of summing node 115 is disabled.
Track following block 112 is enabled when reading from or writing to a track so that the heads track the center line of the data track. Track following is enabled when the PES is small enough for the track follow system to acquire the track. For example, if the amplitude of PES is less than or equal to three quarters of a track, track following is enabled. Track following is also used at the end of a seek operation to correct for any offsets in desired track position and actual track position. In track following mode, block 112 is coupled to switch 113 and the seek block 110 is disabled.
The normal component 108 is also provided as an input to summing node 115. The output of error recovery block 111 or integrating block 112 (depending on the mode of operation) is coupled through switch 113 to summing node 115 on line 114. The output 116 of summing node 115 is provided to compensating block 117 which is a real zero. The output 118 of compensating block 117 is provided as an input to low pass filter 119. The output 120 of low pass filter 119 is coupled to summing node 121 along with the output of seek block 110.
The output 122 of summing node 121 is coupled to transimpedance (or current-controlled) power amplifier 123. The output 124 of amplifier 123 is provided to actuator control block 125. The output 103 of actuator control block 125 is the actual head position signal X.sub.head 103.
In typical disk drive operation, the actual head position at the end of a seek may be offset from the desired track position by some fraction of a track or by one or more tracks. At the end of a seek operation, actual position is determined and compared to ideal head position. This comparison is used to generate a position error signal that is used to generate an actuator drive signal. The disk drive uses this PES as input to the track following circuitry to cause the actuator to move the head to the desired location. The track following compensation is shown in detail in FIG. 2.
In FIG. 2, the normal component 108 of the PES signal is provided to the micro-processor 109 and to a gain block 201. In the example of FIG. 2, the gain block has a gain of one. The normal component 108 is provided to the track following block 112 of the microprocessor 109. This track following block is an integrator plus an A/D and D/A with a transfer function of k/s. The output 114 of the integrating block 112 is provided to summing node 115 along with the output 202 of gain block 201. The output 116 of the summing node 115 is provided to compensating block 117.
Compensating block 117 has a transfer function of (s/z+1). The output 118 of the compensating block 117 is provided to low pass filter 119. The output of low pass filter 119 is output 120. The low pass filter 119 has a transfer function of: ##EQU1## The track following block 112 is used to generate a correction signal so that the read/write head can be moved to the center line of the desired track in the presence of external bias forces.
Theoretically, once the heads have been positioned over a track and have been brought to zero velocity after a seek, the actuator motor does not require any current to hold the heads in place. In actual operation however, windage forces and biases, such as caused by a flex circuit on an actuator arm head assembly, can cause biasing forces on the heads, so that some actuator current is required to maintain head position. These forces can vary with radial head position. Because of this, the current required to keep the actuator in place at different radial positions on the disk drive varies. To compensate for these variations, a diagnostic program is executed to determine and calibrate the current required to hold the actuator in place at different track locations. Typically, these track locations are divided into zones and a calibrated value is stored for each zone in a look-up table. When a seek to a particular zone is performed, the calibrated value is retrieved for that zone and the integration block 112 is initialized to a certain state to compensate for the biasing forces at that location.
When the track seek is to a particular zone, the microprocessor 109 initializes the integrator 112 to this predetermined value. Then, any corrected value indicated by the PES is superimposed on this value so that the read/write heads are moved to the correct positions. This initialization of the integrator is a step function. This scheme results in a disadvantage when the output of the summing node 115 is provided to the zero block 117. The effect of the initialization step function is to provide a large step response that adversely affects head position.
A graph showing head position versus time for a step of the integrator output is illustrated in FIG. 3. The curve 301 represents the response for a unitary input. At peak 302, the response is 60 times the input. At peak 303, the response is minus 20 times the input. This ringing effect continues at peaks 304, 305, 306, etc., until settling occurs (approximately 2.5 milliseconds.) This large response causes the ringing illustrated in FIG. 3 and extends the settling time.
Another disadvantage of prior art control systems is referred to herein as "first sample velocity correction". At the end of a seek operation, head velocity is zero and therefore does not need to be corrected. However, in the first sample after the head comes to the stop, the PES includes correction for a velocity error. This causes an overshoot in the response output.
A number of prior art attempts have been made to provide actuator control systems. Alaimo, U.S. Pat. No. 4,488,187, discloses a digital servo mechanism for read/write head positioning. This mechanism allows for accurate positioning including temperature compensation with a minimum of components. This is accomplished through a different servo pattern. The demodulator is implemented digitally.
Stephens, U.S. Pat. No. 4,575,776, discloses a servo mechanism that incorporates a model of the voice coil motor so that the position error signal can be simulated from a sampled or intermittent error signal. This implementation allows the use of a sector servo scheme instead of dedicated servo patterns.
The invention in Wallis, U.S. Pat. No. 4,594,622, is directed toward the prediction of data track eccentricity and the incorporation of this prediction into the feedback loop of the servo. This modification allows for more accurate and more responsive track following.
Workman, U.S. Pat. No. 4,616,276, is also directed toward measuring track eccentricity and incorporating a prediction of eccentricity into the servo control mechanism. In particular, a method is disclosed for approximating eccentricity in terms of sine functions an for making interactive tracking corrections.
Berti, U.S. Pat. No. 4,616,277, discloses a method for minimizing the mechanical coupling effects of the magnetic head actuators employed on disk drives. In this method, a signal that anticipates and negates the coupling force is provided to a voice coil motor to minimize the response due to the force of a neighboring actuator in operation.
The invention described by Ottesen, U.S. Pat. No. 4,894,599, addresses settling response in single seeks. By incorporating a deadbeat control in the servo, a single track seek can be accurately compensated.
Genheimer, U.S. Pat. No. 4,899,234, describes an adaptable velocity profile for optimizing the many track seek performance of a disk drive. The method disclosed minimizes performance degradation due to variations in temperature, voltage, and actuator characteristics.